The present invention relates to a vapor phase process for the production of difluoromethane, HFC-32. In particular, this invention provides a process for the preparation of HFC-32 that exhibits good product yield and selectivity.
is well known in the art that HFC-32 may be used as a replacement for environmentally disadvantageous chlorofluorocarbon refrigerants, blowing agents, and aerosol propellants. A variety of methods for the vapor phase production of HFC-32 are known.
For example, U.S. Pat. No. 2,745,886 discloses a vapor phase process for fluorinating a variety of halohydrocarbons including methylene chloride, HCC-30, which process utilizes a hydrated chromium fluoride catalyst activated with oxygen. Similarly, U.S. Pat. No. 2,744,148 discloses a halohydrocarbon fluorination process in which an HF-activated alumina catalyst is used.
U.S. Pat. No. 3,862,995 discloses the vapor phase production of HFC-32 by reacting vinyl chloride and HF in the presence of a vanadium derivative catalyst supported on carbon. U.S. Pat. No.4,147,733 discloses a vapor phase reaction for the production of HFC-32 by HCC-30 with HF in the presence of a metal fluoride catalyst.
In practice, these processes for HFC-32 production suffer from a variety of problems including low product yield and selectivity as well as operational difficulties such as feed decomposition. The process of this invention provides for the production of HFC-32 by a process that overcomes some of the disadvantages of the known processes.
The present invention provides a method for HFC-32 production in good yield and selectivity. In general, the process of this invention comprises contacting HCC-30 and HF in the presence of a fluorination catalyst to produce a product stream of difluoromethane, chlorofluoromethane (xe2x80x9cHCFC-31xe2x80x9d), hydrogen chloride, dichloromethane, and hydrogen fluoride and separating HFC-32 from the product stream. In a preferred embodiment, the invention comprises the steps of:
(A) preheating a composition comprising hydrogen fluoride (xe2x80x9cHExe2x80x9d) and HCC-30 and, optionally, HCFC-31, to form a vaporized and superheated composition;
(B) reacting the preheated composition of step (A) in the presence of a fluorination catalyst under conditions suitable to form a product stream comprising HFC-32, HCFC-31 and hydrogen chloride and unreacted HCC-30 and HF;
(C) recovering by distillation from the product stream of step (B) a high boiling fraction comprising HF, HCC-30, and HCFC-31 and a low boiling fraction comprising HFC-32, HCl, HF, and reaction byproducts; and
(D) recovering substantially pure HFC-32 product from the low boiling fraction of step (C).
In step (A) a composition comprising HF and HCC-30 is preheated in at least one vaporizer. By xe2x80x9cpreheating xe2x80x9d is meant to vaporize and superheat the composition. The composition is heated to a temperature of from about 125xc2x0 C. to about 400xc2x0 C., preferably 150xc2x0 C. to about 300xc2x0 C., more preferably from about 175xc2x0 C. to about 275xc2x0 C. and most preferably 200xc2x0 C. to about 250xc2x0 C. The vaporizer, as well as the other vessels used in this process, may be made of any suitable corrosion resistant material.
Although fresh HF and HCC-30 may be used in step (A), preferably the composition of step (A) contains recycled material from step (C) as described below. When the process is run without continuous recycle, the mole ratio of HF to organic, specifically the mole ratio of HF to HCC-30, is from about 1:1 to about 10:1, preferably from about 1:1 to about 4:1. Optionally, fresh HCFC-31 may be added to the composition of step (A).
Alternatively, a continuous recycle stream of the high boiling fraction obtained in step (C) is recycled to step (A) in which case a large excess of HF to organics is used. In the process of this invention, the higher the HF: organics mole ratio, the higher the yield and selectivity for HFC-32. Correspondingly, a large excess of HF will result in the reduction of HCFC-31 produced as well as the concentration of unreacted HCC-30. Additionally, the use of a large excess of HF will decrease catalyst deactivation rates and result in less decomposition in preheaters and vaporizers, especially when the reaction is conducted at pressures in excess of 3 atmospheres. Generally, a ratio of HF to HCFC-31, as measured after separation of HFC-32 from the product stream, of at least about 25:1 to at least about 300:1, preferably at least about 50:1 to at least about 200:1, and more preferably at least about 75:1 to at least about 150:1 is used.
The preheated composition of step (A) is reacted in step (B) in a vapor phase fluorination reaction to form a product stream mixture. The reaction may proceed in one or more isothermal or adiabatic reactors. When more than one reactor is used, the reactor arrangement is not critical, but a sequential arrangement is preferred. Inter-reactor heating or cooling may be used to obtain the best reactor performance.
The reactor or reactors used in this process are filled with a fluorination catalyst and the organic and HF vapor is allowed to contact the catalyst under conditions suitable to form a reaction mixture. The reactor temperature is maintained at from about 125xc2x0 to about 425xc2x0 C., preferably 150xc2x0 C. to about 300xc2x0 C., more preferably 175xc2x0 C. to about 275xc2x0 C. and most preferably 200xc2x0 C to about 250xc2x0 C. Reactor pressure may be atmospheric, subatmospheric, or superatmospheric. Preferably reactor pressure is maintained at from about 0 psig to about 250 psig. Contact time, the time required for the reactants to pass through the catalyst bed assuming a 100% void catalyst bed, is typically from about 1 to about 120 seconds, preferably from about 2 to 60 seconds, more preferably from about 4 to about 50 seconds, and most preferably from about 5 to about 30 seconds.
Any known vapor phase fluorination catalyst may be used in the process of this invention. Exemplary catalysts include, without limitation, chromium, copper, aluminum, cobalt, magnesium, manganese, zinc, nickel and iron oxides, hydroxides, halides, oxyhalides and inorganic salts thereof, Cr2O3/Al2O3, Cr2O3/AlF3, Cr2O3/carbon, CoCl2/Cr2O3/Al2O3, NiCl2/Cr2O3/Al2O3, CoCl2/AlF3 and NiCl2/AlF3. Additionally, supported metal catalysts such as nickel, cobalt, zinc, iron, and copper supported on chromia, magnesia, or alumina may be used. Chromium oxide/aluminum oxide catalysts are described in U.S. Pat. No. 5,155,082 which is incorporated herein in its entirety. Preferably, chromium oxide, a commercially available catalyst, is used. The chromium oxide may be crystalline or amorphous. Preferably, amorphous chromium oxide is used. The catalyst is used in an amount effective to drive the reaction.
The fluorination catalyst may be, and is preferably, pretreated prior to the introduction of the reaction feed stock. By xe2x80x9cpretreatxe2x80x9d is meant to chemically or physically alter the catalyst in order to create active sites on the catalyst at which the reaction may occur. The catalyst is pretreated by calcining under a flow of inert gas such as nitrogen at a temperature from about 200xc2x0 C. to about 450xc2x0 C. for at least about 1 hour. The catalyst is then exposed to HF alone or in combination with up to about 5 to about 99 weight percent of an inert gas at a temperature from about 200xc2x0 C., to about 450xc2x0 C. for at least about 1 hour. Preferably, the catalyst then undergoes a third pretreatment step in which it is contacted with chlorine gas. Preferably, the chlorine is diluted with from about 60 to about 75% HF and/or from about 20 to about 30% of an inert gas. The chlorine may be passed over the catalyst at a total volume chlorine to total volume catalyst of about 1:3,000 v/v, preferably about 10:1,000 v/v, more preferably about 50:500 v/v. Exposure time may be from about 1 to about 200 hours, preferably 5 to 70 hours, more preferably 10 to 30 hours. The chlorine exposure may be conducted at any temperature and pressure convenient to the fluorination reaction.
The flow of chlorine is discontinued after pretreatment is complete and the feed HF and HCC-30 introduced. A small amount of chlorine, from about 0.1 to about 10 mol percent based on organic content, preferably from about 2 to about 8 mol percent, may be added to the reactor, preferably while the fluorination reaction proceeds, for periods of time from about 1 to about 200 hours, preferably from about 5 to about 70 hours, and more preferably from about 10 to about 25 hours, should the catalyst become deactivated to restore activity.
The product stream produced in step (B) contains reaction products which are HFC-32, HCFC-31, and HCl as well as unreacted feed stock such as HF and HCC-30. The product stream of step (B) is fed into a recycle column in step (C). The recycle column may be any standard distillation column known in the art. The high boiling fraction, or bottom stream, from the recycle column is composed of unreacted HF and HCC-30 and intermediate reactant HCFC-31. Preferably, this mixture is recycled to step (A) after recovery. Further in step (C), a low boiling fraction, or top stream, of HFC-32, HCl, HF, and reaction byproducts is recovered.
Alternatively, step (C) may be performed in two parts. In the first part, the product stream of step (B) is quenched. By xe2x80x9cquenchingxe2x80x9d is meant that the temperature of the reaction mixture is reduced to below its dew point. Quenching may be conducted in a packed column containing any suitable corrosion resistant packing material and a suitable refluxing liquid such as HF, HCC-30, and/or HCFC-31 after which the quenched product is fed into the recycle column.
In step (D), substantially pure HFC-32 is recovered from the low boiling fraction of step (C) by any method well known in the art. Preferably step (D) is performed by a series of substeps including step (E), treating the gaseous mixture in an HCl distillation column or aqueous HCl absorption tower under conditions suitable to remove HCl and trace HF. The crude HFC-32 product of step (E) is then treated in step (F) with a first caustic scrubber under conditions suitable to form a neutralized product by neutralizing residual acidity. Typically, the caustic scrubber contains water, sodium hydroxide, or potassium hydroxide. Step (F) is followed by step (G) in which the step (F) product is treated in a second caustic scrubber, preferably comprising sodium hydroxide together with a sulfite, such as sodium sulfite under conditions suitable to remove residual chlorine and form a substantially chlorine-free product. In step (H), the step (G) product is treated with a sulfuric acid scrubber followed by a solid desiccant, such as any suitable, commercially available, molecular sieve that absorbs residual moisture from the gas stream to form a substantially moisture-free product. This is followed by step (I) in which the step (H) product is conducted through a plurality of distillation columns under conditions sufficient to remove the residual impurities and produce substantially pure HFC-32, greater than 99.97 weight percent. Any residual HCFC-31 removed in step (I) may be recycled to step (A).